1. Field of the Invention
The present invention relates to a recording medium which is suitable for recording in high density with a large recording capacity, and yet allows rewriting. The present invention also relates to a recording method and a record-readout method employing the recording medium.
2. Related Background Art
In recent years, applications of memory materials are the most important in electronic industries such as computers and related apparatuses, video discs, and digital audio discs. The research and development of the memory materials are extensively being conducted.
The performances required for memory materials depend on the application field thereof. Generally, high density and large recording capacity are desired.
Heretofore, semiconductor memory materials and magnetic memory materials employing a semiconductor of a magnetic substance have mainly been used for the recording mediums. Recently, however, as the results of progress in laser technique, inexpensive and high-density recording mediums utilizing optical memory materials have come to be practical which employ an organic thin film such as of an organic pigment, and a photopolymer, and the like.
The optical memory makes feasible high density recording and readout at a .mu.m level by utilizing recesses and projection of the surface, or difference in reflectivity of the surface of the recording medium. Such recording mediums utilize a thin film of a metal or a metal compound, a thin film of an organic pigment, or the like, on which information is recorded by boring holes or changing the reflectivity by evaporation or fusion by means of heat of laser beams. In these methods, the recording density depends on the spot diameter of the laser beam employed.
At the moment, however, with rapid advance of processing images as information, high-density and large-capacity memory by a smaller size of the apparatus is desired to be developed.
Recently, scanning tunnel microscopy (hereinafter referred to as "STM") has been developed, which makes it feasible to observe directly the electronic structure of atoms on the surface of conductors [G. Binning et al., Helvetica Physica Acta, 55 726 (1982)]. By the STM, a real spatial image, whether of a single crystal or of an amorphous matter, can be measured advantageously without impairing the medium by electric current and with low electric power. Further the STM observation may be conducted in an atmospheric environment and may be done for various materials. Therefore the STM is promising for a variety of application fields.
The STM utilizes tunnel current which flows when a metal probe and an electroconductive substance under application of an electric voltage therebetween are brought close to about 1 nm. This tunnel current is extremely sensitive to the change of the distance between the two. Therefore the scanning with the probe by keeping the tunnel current constant can represent the real spatial surface structure of a material, and can simultaneously give various information regarding the entire electron cloud of the surface atoms.
The analysis by the STM has been applicable only to electroconductive samples. However, the STM has recently come to be employed for analysis of structure of monomolecular films formed extremely thin on the surface of an electroconductive material. Therefore the STM is expected to be useful as a reproduction technique in high-density record by utilizing the difference of states of individual organic molecules.
The application of STM to recording and reproduction technique includes a method in which recording is conducted by changing the surface state of a recording medium by use of electron beam, ion beam, or electromagnetic wave such as X ray and light, and the record is reproduced by STM; and a method in which recording and reproduction are conducted by means of STM by use of a recording medium exhibiting memory effect in voltage current characteristics, such as a thin film of a chalcogenated substance and a thin film of a .pi.-electronic organic compound.
Further, a trial is made to form cone-like projections on the surface of an Rh-Zr alloy sample by local fusion by application of field-emission voltage to a probe electrode of STM. [U. Staler et al., Appl. Phys. Lett., 51 (4) 27 July 1987]
Electrostatic recording methods, in which a latent image is formed by electric discharge or electric current flow by use of a needle-shaped electrode are well known, and are applied in various ways to recording on mediums such as recording paper (Japanese Patent Application Laid-open No. 49-3435).
The thin film employed in such electrostatic recording mediums has a thickness of a .mu.m level. No report is found on electrostatic readout and reproduction of the latent image on the recording medium.